Device and method for regenerating an electroless metal plating bath

ABSTRACT

In regenerating an electroless metal plating bath by electrodialysis, it has been found that the metal of the metal plating bath deposits in the electrolysis arrangement. To overcome this problem, an improvement to prior art regenerating devices is suggested, said improvement consisting in providing main cation exchangers for removing ions of this metal from a concentrate fluid. The main cation exchangers are coupled to the concentrate compartments of the electrolysis arrangement in such a manner that the concentrate fluid flowing through the concentrate compartments is allowed to pass through the main cation exchangers and to be recirculated back into the concentrate compartments.

The invention relates to a device and a method for regenerating anelectroless metal plating bath, said metal plating bath morespecifically containing hypophosphite. The invention more specificallyserves to regenerate baths intended for the electroless deposition of anickel layer, more specifically of a nickel-phosphorus layer, byelectrodialysis.

The electroless plating on substrates of metals and alloys is anautocatalytic process by which metal ions in solution are reduced tometal by means of a reducing agent contained in the solution and aredeposited onto a suitable substrate. Other components such as phosphorusare often incorporated in the layer.

Usually, such type of method is used for depositing metals such asnickel, copper, cobalt, palladium, platinum and gold onto a substrate.In most cases, the reducing agents used are sodium hypophosphite, sodiumboron hydride or dimethyl amino borane.

As compared to the conventional metal electroplating methods used fordepositing metal, the electroless deposited layers offer a series ofadvantages that include homogenous coating distribution, advantageousmechanical properties and high corrosion resistance.

By way of example, the method in accordance with the invention will bedescribed herein after for depositing a nickel-phosphorus layer usinghypophosphite.

The method can also be utilized for other electroless metal platingprocesses, though:

The essential process in electroless nickel plating is represented bythe following equation:Ni²⁺+2H₂PO₂ ⁻+2H₂O−>Ni+H₂+2H⁺+2H₂PO₃ ⁻  (1)

Accordingly, during electroless nickel plating, dissolved nickel ionsand the hypophosphite reducing agent are permanently used up with theconcentration of the oxidation product orthophosphite (H₂PO₃ ⁻, HPO₃ ⁻⁻)increasing. In the bath, the nickel and hypophosphite concentration mustbe kept within a narrow range. These constituents must thereforecontinuously be replenished. The metal ions are replenished in the formof salts, leaving the bath enriched in interfering anions such assulfate.

Since the reducing agent and the salts containing the nickel ions alsoform reaction products during the plating reaction, said reactionproducts accumulating in the plating bath, the useful life of the bathis inevitably limited. The bath age is usually indicated in MetalTurnover (MTO). 1 MTO is achieved once the entire amount of the normalinitial metal concentration has been deposited from one unit of volume.After usually 6-10 MTO, the interfering substances have reached such ahigh concentration that the quality and deposition rate of the metal areno longer within tolerable ranges. According to prior art, these bathsmust then be discarded and a new bath must be prepared.

The need for discarding baths and for preparing new ones involves highcosts and increases the environmental impact. Various methods havetherefore been proposed for extending the useful life of electrolessmetal plating baths.

U.S. Pat. No. 5,221,328 describes a method by means of which, in anickel-phosphorus plating bath, orthophosphite is caused to precipitatein the form of a metal salt and is allowed to be removed for the purposeof extending the useful life of an electroless nickel bath. Yttrium andlanthanides may be employed as precipitating agents. The chemicals usedfor the purpose are quite expensive, though. Furthermore, constituentsof these additives, which remain in solution in the bath, may affect thequality of the metal coatings.

In “Plating and Surface Finishing”, September 1995, pages 77-82, C. D.lacovangelo suggests to prevent the nuisance of nickel orthophosphiteprecipitates by adding complexing agents. The concentration of dissolvedfree nickel ions is reduced as a result thereof.

In the ENVIRO CP method of Martin Marietta, U.S.A, the interferingconstituents are removed by adsorption onto ion exchange resins. Forcomplete removal and regeneration of the plating bath, a complicatedmethod necessitating several different ion exchange columns and tanksfor various processing fluids is performed.

Another possibility for regenerating electroless nickel baths consistsin a method making use of electrodialysis. In the method usingelectrodialysis, charged ions are transported (transferred) in anelectric field through a permselective ion exchange membrane so that theions of active substances can be suitably separated from the ions ofinterfering substances.

Y. Kuboi and R. Takeshita describe a method using electrodialysis forremoving the undesirable bath constituents (Electroless NickelConference 1989, Proceedings, Prod. Finishing Magazine, 1989, pages 16-1through 16-15). By this method, the electroless nickel bath is passedthrough an electrodialysis cell in the form of what is termed a diluate.For this purpose, on the anode side, the diluate compartment in theelectrodialysis cell is separated from the anode compartment contactingthe anode by an anion exchange membrane and on the cathode side from thecathode compartment contacting the cathode by a cation exchangemembrane. These last two compartments are also termed concentratecompartments. The undesirable sulfate and orthophosphite ions in theplating bath are transferred into the anode compartment and theundesirable sodium ions, which originate from the sodium hypophosphiteutilized, into the cathode compartment. Laboratory tests however showedthat not only the undesirable sulfate, orthophosphite and sodium ionsare transferred to the concentrate compartments, but the bathconstituents that are important for the plating method as well, namelythe nickel and hypophosphite ions and the organic complexing agents(mostly carboxylic acids or the anions thereof).

DE 43 10 366 C1 describes a method for regenerating electrolessnickel-phosphorus baths by electrodialysis. For this purpose, thenickel-phosphorus bath to be regenerated is passed through a compartmentin an electrodialysis cell, said compartment being separated from theadjacent compartments by an anion exchange membrane both on the side ofthe anode and on the side of the cathode (diluate compartment). Byapplying an electric field, ortho- and hypophosphite ions aretransferred to the concentrate compartment located with the anode sideturned towards the diluate compartment. Next, this solution is deliveredto the cathode compartment which contacts the cathode. From there,hypophosphite is allowed to be transferred back into the diluatecompartment while orthophosphite is reduced to hypophosphite at thecathode, with the thus obtained hypophosphite being described to betransferred next into the diluate compartment. Tests however showed thatthis reduction reaction does not happen as a matter of fact. It furthersuggests to connect in parallel a plurality of the cells mentioned. Thiscell is not capable of overcoming the drawback inherent to the methoddescribed by Y. Kuboi and R. Takeshita. This solution is furthermoreenriched in sulfate and sodium ions.

U.S. Pat. No. 5,419,821 also describes an electrodialytic process forregenerating an electroless metal plating bath. In a manner similar tothat described in DE 43 10 366 C1, hypophosphite and orthophosphite aretransferred through an anion exchange membrane to a concentratecompartment located on the anode side and are removed as a resultthereof. In this case as well, the concentrate solution on the anodeside is transferred to the cathode compartment in order to allowhypophosphite to return from there to the diluate compartment. Byaddition of magnesium or calcium salts to the solution that iscirculated across said compartment, orthophosphite is precipitated, thusbeing removed from the overall process. The disadvantage thereof howeveris that interfering sodium and sulfate ions cannot be removed from thenickel bath solution.

In an effort to overcome the drawbacks of the methods described hereinabove, EP 0 787 829 A1 suggests a method of regenerating electrolessnickel-phosphorus baths by electrodialysis, with the method beingutilized in two different variants. In either of the two variants, thismethod is performed discontinuously. The one variant constitutes atwo-stage method by which the spent plating solution is first conductedinto the diluate compartment of an electrodialysis cell that is definedagainst two concentrate compartments by an anion exchange membrane onthe side facing the anode and by a monoselective cation exchangemembrane on the side facing the cathode. Monoselective ion exchangemembranes differ from normal ion exchange membranes in that they allowsingly charged ions to pass through, not however multiple charged ions.In the first stage of the process, ions of sodium, hypophosphite,orthophosphite, sulfate and carboxylic acid anions are transferred intothe neighbouring compartments whereas the nickel ions remain in thediluate compartment. Next, the respective solutions are conducted into asecond electrodialysis cell accommodating, between two diluatecompartments, a concentrate compartment that is separated from thesediluate compartments by a monoselective anion exchange membrane on theanode side thereof and by a cation exchange membrane on the cathode sidethereof. In this event, the anions of hypophosphite and carboxylic acidand the cations of sodium are transferred back into the diluatecompartment, not however the ions of orthophosphite and sulfate. As aresult, the ions of orthophosphite and of sulfate are removed, nothowever the sodium ions. Since charge balance must be ensured in eitherof the various stages of the process, it is not possible to remove thetotal amount of the ions of orthophosphite and sulfate since the portionof anionic counter ions that corresponds to the sodium ions remaining inthe diluate compartment must also remain in the diluate compartment.This substantially affects removal efficiency.

In the second variant, which is conceived as a one-stage method, thesolution of the bath is filled into the cathode compartment of anelectrodialysis cell consisting of three electrolyte compartments, thecentral compartment thereof being separated from the other compartmentsby a monoselective anion exchange membrane on its anode side and by amonoselective cation exchange membrane on its cathode side. The solutioncontained in the anode compartment is conducted into the cathodecompartment. The solution of the bath is first introduced into thecathode compartment. Ions of hypophosphite and of orthophosphite aredescribed to be transferred into the central compartment. This seemsimpossible though since a cation exchange membrane is disposed betweenthe two compartments. For this reason, it is not clear how the methodcan be performed.

DE 198 49 278 C1 further describes a method and a device forelectrodialytically regenerating an electroless metal plating bathcontaining a reducing agent in the form of hypophosphite ions thatensure a constant low percentage of interfering ions in the metalplating bath. The user of this invention can extend almost ad lib theuseful life of the baths. In practice, far more than 200 MTO have beenachieved heretobefore. In addition to extending the useful life, theyalso ensure that consistent high quality coatings be deposited. Thesuitable arrangement of the anion exchange membranes and of themonovalent permselective anion exchange membranes described in DE 198 49278 C1 permits to remove the monovalent anions (more specifically thehypophosphite) from the circuit of the waste substances and torecirculate them back into the circuit of the active substances.

The known methods and devices suffer from various differentdisadvantages:

1. Depending on the type of membrane used, the amount of metal ions fromthe circuit of active substances (diluate) lost to the circuit of wastesubstances (concentrate) can be as much as 10% of the amount depositedin the metal plating bath. Active substances get lost as a resultthereof.

2. The drop-out current contains a considerable amount of metal ionssuch as nickel ions which accordingly requires further waste treatmentexpense and leads to the formation of a corresponding amount of metalslurry.

3. The major drawback of the known method and device is that undesirablemetal precipitates form within the regeneration device. On the one side,this reduces the availability of the plant which inevitably has to besubjected to cleaning cycles (metal stripping) and on the other sidecauses damages and an efficiency loss to the plant.

It is therefore an object of the present invention to avoid thedisadvantages of the known methods and devices and more specifically tofind a method and a device permitting to regenerate metal plating bathsmore specifically comprising a hypophosphite reducing agent. Theinvention is more specifically intended to achieve that the activesubstances (metal ions, reducing agent, complexing agent) be largelymaintained in the circuit of active substances and that the interferingsubstances (reaction products, inerts) be removed to the largestpossible extent from the circuit of active substances.

In overcoming the problem, the invention provides the device forregenerating an electroless metal plating bath according to claim 1 andthe method for regenerating an electroless metal plating bath accordingto claim 8. Preferred embodiments of the invention are recited in thesubordinate claims.

Any electrodialysis arrangement/s, diluate compartment/s, concentratecompartment/s, main cation exchanger/s, anion exchanger/s, ion exchangemembrane/s, cathode/s, anode/s, current supply/s, collecting tank/s,regenerant fluid vessel/s, service reservoir/s, safety cationexchanger/s or the like is to be construed in the following descriptionof the invention and in the patent claims as one or several suchelements.

The device and the method in accordance with the invention mainly serveto regenerate by electrodialysis an electroless metal plating bath thatmore specifically contains hypophosphite, for example a bath fordepositing layers of nickel, cobalt, copper, palladium, platinum orgold. The device and the method are more specifically suited forelectrodialytically regenerating electroless nickel baths. Morespecifically, the baths adapted to be regenerated in accordance with theinvention may comprise a hypophosphite reducing agent. Therefore,phosphorus can also be deposited as a constituent component of thelayer. All of the hypophosphite salts and the free acid H₃PO₂ may beemployed as hypophosphite. The salts utilized may more specifically beutilized in the form of alkali salt, alkaline earth salt and ammoniumsalt.

The device in accordance with the invention comprises electrodialysisarrangements each comprising diluate compartments for receiving themetal plating bath, concentrate compartments for receiving a concentratefluid serving to incorporating the interfering substances removed fromthe metal plating bath, said concentrate compartments each beingseparated from the diluate compartments by ion exchange membranes, andanodes and cathodes. Further, the device additionally comprises maincation exchangers for removing metal ions from the concentrate fluid,said cation exchangers communicating with the concentrate compartmentsin such a manner that the concentrate fluid is passed through the maincation exchangers and may be circulated back to the concentratecompartments. For electrodialytic treatment, the metal plating bath maybe passed through the diluate compartments in the electrodialysisarrangements and the concentrate fluid through the concentratecompartments in the electrodialysis arrangements.

In addition to the electrodialysis arrangements an ion exchanger system(main cation exchanger) is thus coupled in the inventive manner to theregenerating system so as to allow concentrate fluid to flow through themain cation exchanger. The concentrate fluid is supplied to one orseveral main columns (main cation exchangers) comprising the ionexchanger resin. The ion exchanger resin is a cation exchanger resin.Such type resins are commercially available, for example from Bayer,Germany (Lewatit® types). The cation exchanger resin binds the metalions, for example nickel or copper ions, and exchanges them for H₃O⁺— orfor sodium ions. Once the concentrate fluid has been passed through themain cation exchanger, it is circulated back to the concentratecompartments of the electrodialysis arrangement.

Due to the invention, a loss of metal ions from the circuit of activesubstances (diluate) to the circuit of waste substances (concentrate)will not lead to the disadvantages described. By removing the metal ionsfrom the concentrate, the following advantages are achieved:

1. By removing the metal ions from the concentrate by means of cationexchange, the metal ions, for example the nickel ions, concentrate inthe main cation exchanger. As a result thereof, the metal ionstransferred to the concentrate can be recycled and be recirculated backinto the circuit of the active substances. The loss of active substancesis thus minimized.

2. By removing the metal ions from the concentrate, the cost of wastewater treatment may be reduced as well since the treatment using themain cation exchanger is much less complicated than a conventional wastewater treatment which additionally needs considerable amounts ofchemicals for precipitating the metal ions from the bath. In many cases,it is absolutely impossible to remove the metal ions from theconcentrate solution as they may contain considerable amounts ofcomplexing agents. Removal of the metal ions from the concentrateminimizes the environmental impact.

3. By removing the metal ions from the concentrate, the metal is furtherprevented from precipitating by plating in the regeneration device. Theavailability of the plant is considerably increased as a result thereofas otherwise inevitable cleaning cycles (metal stripping) will benecessary. The plant is moreover subjected to less wear.

It has been tested whether the concentrate can be enriched instabilizing agents in order to at least delay plating of theelectrodialysis arrangements. Usually, such type stabilizing agents areadded to electroless metal plating baths in an effort to prevent metalfrom undesirably precipitating in the bath tank and on the insertstherein. For nickel baths, low concentrations of lead compounds are usedfor example.

It has been found out however that stabilization is disadvantageousbecause part of these substances gets into the diluate where they cannegatively affect the quality and deposition performance of the bath.This is particularly true for baths that are stabilized at low levelsand that serve to deposit nickel layers with high phosphorus content.The use of these stabilizing agents is also disadvantageous because theymake waste water treatment more difficult.

The invention also permits to profitably regenerate baths with lowthroughput as they are currently used in practice by means ofelectrodialysis. As a result thereof the quality of the layers can bekept on a constant and optional level in these cases as well.

Preferably, the method can be performed continuously, i.e., regenerationis carried out without any interruption for maintenance works for a verylong period of time, for example for one or several months.

The concentrate fluid in the electrodialysis arrangement is conductedthrough the ion exchanger in a manner in accordance with the inventionin order to remove the metal ions that have entered the concentratefluid. The metal concentration that builds up in the concentrate circuitcan be regulated by the size of the volume stream V_(ix) of concentratefluid flowing across the main cation exchanger. In theory, as metal ionsare permanently transferred from the diluate fluid to the concentratefluid, an infinitely large volume stream V_(ix) is needed in order tocause the concentration of the metal ions in the concentrate fluid toreduce to nearly zero. Therefore, the concentration of the metal ions isadjusted so as to reliably prevent metal from plating theelectrodialysis arrangement. The nickel ion concentration is less than800 mg/l, the upper limit of the still tolerable nickel concentrationdepending on the temperature of the concentrate fluid in theelectrodialysis arrangement, on the pH value, on the concentration ofthe reducing agents (hypophosphite) and on other parameters and beingadapted to be determined separately by way of experiment.

In a particularly advantageous embodiment, the device in accordance withthe invention comprises collecting tanks that communicate with theconcentrate compartments and with the main cation exchangers in such amanner that the concentrate fluid is allowed to circulate in a firstcircuit between the concentrate compartments and the collecting tanksand in a second circuit between the collecting tanks and the main cationexchangers.

This arrangement permits the formation of two fluid circuits that can becontrolled independent of each other. On the one hand, the volumestreams flowing between the electrodialysis arrangements and thecollecting tank on the one side and between the collecting tank and themain cation exchanger on the other side, the latter being referred to asV_(ix), can be adjusted independent of each other. V_(ix) may forexample be adjusted so as to be much smaller than the volume streamflowing between the electrodialysis arrangements and the collectingtank. The concentration of the metal ions in the concentrate fluid canbe directly influenced in a simple manner by adjusting the ratio ofthese volume streams. On the other hand, if necessary, the temperaturesof the volume streams can also be set to different values.

A device in accordance with the invention that has the followingfeatures has been found to be advantageous:

-   -   a) a first electrodialysis arrangement alternatingly comprising        first concentrate compartments and first diluate compartments as        well as cathodes and anodes, each of said diluate compartments        being separated from a respective neighbouring concentrate        compartment located on the cathode side of the diluate        compartment by a monoselective cation exchanger membrane and        from a respective neighbouring concentrate compartment located        on the anode side of the diluate compartment by an anion        exchanger membrane,    -   b) a second electrodialysis arrangement alternatingly comprising        second diluate compartments and second concentrate compartments        as well as cathodes and anodes, each of said concentrate        compartments being separated from a respective neighbouring        diluate compartment located on the cathode side of the        concentrate compartment by an anion exchanger membrane and from        a respective neighbouring diluate compartment located on the        anode side of the concentrate compartment by a monoselective        anion exchanger membrane.

The metal plating bath is concurrently conducted through all the firstand second diluate compartments in the two electrodialysis arrangementsthat are hydraulically connected in parallel. Likewise, the concentratefluid is concurrently conducted through all the first and secondconcentrate compartments in the two electrodialysis arrangements thatare hydraulically connected in parallel.

The concentrate compartments and the diluate compartments arealternatingly arranged in the two electrodialysis arrangements.

Further,

-   -   c) current supplies for the cathodes and the anodes of the first        and second electrodialysis arrangements are provided for in this        device.

In a very simple embodiment, the electrodialysis arrangement is providedwith the following arrangement features:

-   -   a) a first electrodialysis arrangement, comprising two first        concentrate compartments and one first diluate compartment        disposed therein between, said compartments being employed as        electrolyte compartments, with the diluate compartment being        separated on the cathode side thereof from one of the        concentrate compartments by a monoselective cation exchanger        membrane and on the anode side thereof from the other        concentrate compartment by an anion exchanger membrane,    -   b) a second electrodialysis arrangement, comprising two second        diluate compartments and one second concentrate compartment        disposed therein between, said compartments being employed as        electrolyte compartments, with the concentrate compartment being        separated on the cathode side thereof from one of the diluate        compartments by an anion exchanger membrane and on the anode        side thereof from the other diluate compartment by a        monoselective anion exchanger membrane,    -   c) at least one cathode and at least one anode being provided in        each electrodialysis arrangement and    -   d) a current supply for the cathodes and the anodes.

Instead of but three electrolyte compartments (diluate compartments,concentrate compartments), more than three electrolyte compartments maybe provided for in each electrodialysis arrangement, the respective onesof the diluate and concentrate compartments being disposed alternatinglyand being separated by ion exchanger membranes in compliance with theabove mentioned requirement. With the ion exchanger membranes havinggiven dimensions, a sufficiently large exchange surface for the spentmetal plating bath is thus made available in the membranes. The largerthis exchange surface, the faster and more efficient the regenerationprocess of the bath. Therefore, in an optimum configuration of theregeneration array, a plurality of diluate and concentrate compartmentsis disposed in alternating sequence in both the first and the secondelectrodialysis arrangement. Two stacks of electrolyte cells throughwhich the diluate fluid is conducted across the diluate compartments andthe concentrate fluid across the concentrate compartments are thusobtained. In principle, the two electrodialysis stacks need not have thesame number of electrolyte compartments. It may for example beadvantageous to provide the first electrodialysis arrangement with agreater number of diluate and concentrate compartments than the secondelectrodialysis arrangement.

Through the special arrangement of the ion exchanger membranes, thefirst concentrate compartments in the first electrodialysis arrangementare defined by anion exchanger membranes on the cathode side of thiscompartment arid by monoselective cation exchanger membranes on theanode side of this compartment. The anode and the cathode are disposedon the end faces of the electrodialysis stack. Unlike the given sequenceof membranes separating the respective compartments, the electrolytecompartments contacting the cathode and the anode are separated from theadjacent electrolyte compartments by cation exchange membranes. Theseouter electrolyte compartments hold an electrochemically inertconducting salt solution that is delivered across the two compartmentsin the circuit, for example a sodium sulfate solution. Undesirableelectrode reactions, which would destroy the electrodes or lead to theformation of further undesirable reaction products on the electrodes,are thus prevented from happening in these compartments.

Likewise, the second concentrate compartments in the secondelectrodialysis arrangement are bounded by anion exchange membranes onthe cathode side thereof and by monoselective anion exchange membraneson the anode side thereof. Again, one anode and one cathode are disposedon the end sides of this second electrodialysis stack. Unlike the givensequence of membranes separating the diluate compartments and theconcentrate compartments, the electrolyte compartments contacting thecathode and the anode are separated from the adjacent electrolytecompartments by cation exchange membranes. In this second case as well,suited inert solutions are contained in the cathode and in the anodecompartment so that undesirable electrode reactions are prevented fromhappening.

The surface ratio of the normal anion exchange membranes to themonoselective anion exchange membranes in the two electrodialysis stacksand the pH value of the solution conducted through the concentratecompartments (preferably about 8.5) determine the degree of loss ofanionic active substances, meaning of hypophosphite and carboxylic acidanions.

The first electrodialysis arrangement and the second electrodialysisarrangement can be combined in one common electrodialysis stack and maybe disposed in such a manner that one cathode is disposed on one endface of the common electrodialysis stack and that one anode is disposedon the other end face thereof. For this purpose, the respective stacksare not electrically isolated from each other. For this purpose there israther provided an anion exchange membrane on the interface between thetwo stacks for separating the last concentrate compartment in the firstelectrodialysis arrangement on its cathode side from the last diluatecompartment in the second electrodialysis arrangement on its anode side.In this case, the cathode compartment provided for on the lastelectrolyte compartments and the corresponding anode compartment as wellas the associated electrodes are dispensed with. Accordingly, but onecathode compartment and one anode compartment as well as one cathode andone anode are provided for on the end faces of the stack in this case.

Further, in another embodiment, the first electrodialysis arrangementand the second electrodialysis arrangement can be combined into onecommon electrodialysis stack in such a manner that the electrolytecompartments in the electrodialysis arrangements that are turned towardthe cathode are oriented toward the respective other stack ofelectrodialysis cells. One common cathode is disposed between the twoelectrodialysis arrangements and one anode is disposed on either endface of the common electrodialysis stack. This combination has theadvantage that only one stack must be realized. In this case, twocurrent supplies are provided, namely one current supply for the cathodeand the one anode and another current supply for the cathode and theother anode. The electric circuits of the two electrodialysisarrangements can of course also be connected in parallel so that onecurrent supply will do.

In an alternative embodiment, the various electrolyte compartments arearranged in a reverse sequence. In this case, the electrolytecompartments in the electrodialysis arrangements that are turned towardthe anode are oriented toward the respective other stack ofelectrodialysis cells. One common anode is disposed between the twoelectrodialysis arrangements and one cathode is disposed on either endface of the common electrodialysis stack.

The spent bath solution, which, in addition to the active substances ofthe bath, meaning ions of hypophosphite, carboxylic acid and nickel,also contains interfering concomitant substances such as ions oforthophosphite, sulfate and sodium, is supplied simultaneously to allthe diluate compartments of the two electrodialysis arrangements thatare hydraulically connected in parallel. In the first electrodialysisarrangement, all of the anions are transferred from the diluatecompartments into the concentrate compartments that are disposed on theanode side of the diluate compartments and the sodium ions aretransferred to the concentrate compartments that are disposed on thecathode side of the diluate compartments, with the nickel ions remainingin the diluate compartments. In the second electrodialysis arrangement,only the monovalent anions, that is the ions of hypophosphite andcarboxylic acid, are transferred from the concentrate compartments intothe diluate compartments located on the anode side of the concentratecompartments, the cations held in the concentrate compartments and thebivalent anions, namely the ions of orthophosphite and sulfate,remaining in these compartments in this case.

By utilizing, on the cathode side of the diluate compartment of thefirst electrodialysis arrangement, a monoselective cation exchangemembrane, sodium ions are virtually selectively transferred from thediluate compartment into the concentrate compartment. Except for smalllosses, the special arrangement of the membranes does not allow nickelions to pass from the diluate compartment into the concentratecompartment. By further utilizing, in both electrodialysis arrangements,on the anode side of the diluate compartment, an anion exchangemembrane, not only hypophosphite but orthophosphite and sulfate as wellare transferred from the diluate compartment into the concentratecompartment. The loss of ions of hypophosphite and carboxylic acid lostto the diluate compartment is selectively compensated for in disposing,in the second electrodialysis arrangement, a monoselective anionexchange membrane on the anode side of the concentrate compartment sothat these ions are selectively transferred from the concentratecompartment into the diluate compartment.

As a result, with the solution being continuously passed through the twoelectrodialysis arrangements, the ions of sodium, orthophosphite andsulfate are mainly removed from the spent solution whereas the activesubstances are retained therein. Accordingly, the method and the devicein accordance with the invention permit to achieve the optimalefficiency in removing interfering bath constituents thus providing thesolution of the problem the invention aimed at resolving.

Since the two electrodialysis arrangements are hydraulically operated inparallel and not in series, electroneutrality must be preserved for thetransfer of ions within the entire arrangement only. Meaning, the amountof anionic substances passing across the membranes in the anodicdirection needs only equal the amount of cationic substances passing themembranes in the cathodic direction with regard to the arrangement as awhole. The metal plating bath is permanently and continuously passedthrough the two electrodialysis arrangements so that, in continuousoperation, a balance is maintained in which the interfering substancesare being largely removed.

The concentrate fluid flows through the concentrate compartments. Saidconcentrate fluid is enriched in the interfering substances removed fromthe spent metal plating bath and carries entrained water. In order forthe concentration of these interfering substances not to exceed acritical value, the concentrate fluid is diluted constantly or at leastfrom time to time (intermittently). Moreover, it is possible to addsodium hydroxide to this fluid. This addition permits efficientseparation of the orthophosphite ions from the hypophosphite ions inthat the pH of the concentrate fluid is optimally adjusted to about 8.5(formation of HPO₃ ⁻⁻ out of H₂PO₂ ⁻).

When the device starts operation, the main cation exchangers are chargedwith H₃O⁺ or with sodium ions, depending on the type of cation exchangerused. In operation, the main cation exchangers are gradually chargedwith metal ions. Once a certain charge, which may vary as a function ofthe type of exchanger used, of the main cation exchangers is achieved,the main cation exchangers will no longer adsorb further metal ions sothat these can no longer be removed from the concentrate fluid. Ifnecessary, operation must therefore be halted to regenerate the maincation exchangers.

Further, to regenerate the main cation exchangers, first regenerantfluid vessels are provided for holding regenerant fluid intended toregenerate the main cation exchangers, said vessels being coupled to themain cation exchangers. An acid, more specifically sulfuric acid, ispreferably employed as a regenerant fluid. Using an acid, the maincation exchangers, which are charged with metal ions, are again chargedwith H₃O⁺ ions, the metal ions being liberated into the regenerantfluid.

Service reservoirs for the concentrate fluid, which are coupled to thecollecting tanks and the main cation exchangers, are further provided.Safety cation exchangers, which are coupled to the main cationexchangers for post-treatment of the concentrate fluid treated in themain cation exchangers, are also provided. Eventually, there areprovided second regenerant fluid vessels for holding regenerant fluidintended for use in the regeneration of the safety cation exchangers.

The figures, which are indicated as follows, serve to explain theinvention in closer detail.

FIG. 1 gives an overall schematic view of the device in accordance withthe invention;

FIG. 2 gives a schematic view of the partial processes in a preferredelectrodialysis equipment.

FIG. 1 illustrates a metal plating bath tank M that holds for example anelectroless nickel bath containing a hypophosphite reducing agent. Rinsewater can be transferred from a rinse water tank S to the metal platingbath tank M to compensate for evaporation loss.

The metal plating bath is circulated between the tank M and a diluatetank V_(D). The volume stream amounts to 100 l/h for example. The bathis further circulated between the diluate tank V_(D) and anelectrodialysis equipment E, comprising two electrodialysis arrangementsfor example This volume stream amounts for example to 8 m³/h. Inseparating the volume streams flowing from the bath tank M to thediluate tank V_(D) and from the diluate tank V_(D) to theelectrodialysis arrangements E, the metal plating bath, which in mostcases is very hot (for example T=90° C.), can already be electrodialyzedwith little cooling. This is achieved in that the volume streams flowingbetween the diluate tank V_(D) and the electrodialysis arrangements E ismuch smaller than the one flowing between the bath tank M and thediluate tank V_(D).

Diluate and concentrate compartments, which are shown schematically, areprovided for in the electrodialysis arrangements E. This is denoted inFIG. 1 by the vertical partition through the schematically shownelectrodialysis arrangements E, said partition being intended toillustrate that the electrodialysis arrangements E contain a stack ofseveral diluate and concentrate compartments that are arrangedalternatingly. Further, one anode is disposed on the one side and onecathode on the other side of the stack. A preferred embodiment ofelectrodialysis arrangements is shown in FIG. 2 (it will be describedherein after).

The bath flows simultaneously through all of the diluate compartments asthe diluate compartments are hydraulically connected in parallel.Concurrently, a concentrate fluid, which is preferably weakly alkalineand contains, in operation, transferred substances originating from thediluate fluid (for example ions of orthophosphite, sulfate, sodium),flows simultaneously through all of the concentrate compartments in theelectrodialysis arrangements E, said compartments being hydraulicallyconnected in parallel as well. The concentrate fluid also contains smallamounts of transferred nickel ions originating from the metal platingbath.

In the electrodialysis equipment E, ions of orthophosphite, sulfate andsodium in particular are removed from the metal plating bath and enterthe concentrate fluid. Small amounts of nickel and hypophosphite ionsalso pass into the concentrate fluid.

The concentrate fluid is circulated between the electrodialysisarrangements E and a collecting tank V_(K).

The concentrate fluid flowing into the collecting tank V_(K) iscirculated in another fluid circuit to the main cation exchanger I_(X)that is preferably configured to be tubular (in the form of a column).

The main cation exchanger column I_(X) is filled with a cation exchangermaterial. The main cation exchanger I_(X) is charged with nickel ionsthrough the concentrate fluid flowing there through. Concurrently, H₃O⁺ions from the main cation exchanger I_(X) are released to theconcentrate fluid. Since the pH value of the concentrate fluid ispermanently lowered as a result thereof, a base such as NaOH may beadded.

The following devices are further provided.

The concentrate fluid can be temporarily stored in a separate servicereservoir V_(ZK). For this purpose, the service reservoir V_(ZK) iscoupled to the collecting tank V_(K) and the main cation exchangerI_(X). The concentrate fluid can be conducted from the collecting tankV_(K) into the service reservoir V_(ZK) and from there into the maincation exchanger I_(X).

The main cation exchanger I_(X) is further coupled to a first regenerantfluid vessel V_(RS1). The first regenerant fluid tank V_(RS1) serves tohold regenerant fluid. If necessary, the regenerant fluid can also beconducted directly into the metal plating bath, for example if the pHvalue of the bath is to be adjusted.

The main cation exchanger I_(X) is further coupled to a safety cationexchanger I_(S). Both the safety cation exchanger I_(S) and the maincation exchanger I_(X) contain cation exchanger material.

The safety cation exchanger I_(S) is coupled to a second regenerantfluid vessel V_(RS2). Said second regenerant fluid tank V_(RS2) alsoserves to hold regenerant fluid.

Wash water can be conducted both into the main cation exchanger I_(X)and into the safety cation exchanger I_(S). Next, this wash water can betransferred into the metal plating bath.

In removing the nickel ions from the concentrate fluid flowing throughthe main cation exchanger I_(X), the ion exchanger is charged withnickel ions. The ion exchanger will have to be regenerated upon capacityexhaustion thereof. This can be performed in the following manner:

Regeneration Step 1 (Displacement of the Concentrate Fluid):

In a first regeneration step, the concentrate fluid contained in themain cation exchanger I_(X) is displaced by the regenerant fluid storedin the first regenerant fluid vessel V_(RS1). The concentrate fluid isthereby recirculated back into the collecting tank V_(K). For thispurpose, the regenerant fluid is transferred from the first regenerantfluid vessel V_(RS1) into the main cation exchanger I_(X). This way ofproceeding makes sure that the least possible amount of concentrate isintroduced into the regenerant fluid. This method step can be automatedby controlling the volume streams flowing from the first regenerantfluid vessel V_(RS1) to the main cation exchanger I_(X) and from thereinto the collecting tank V_(K) through automated valve switching for aset time or for example through measuring the pH value at the output ofthe main cation exchanger I_(X) to the collecting tank V_(K). In thelatter case, pH sensors detect whether the pH value of the concentratefluid flowing from the main cation exchanger I_(X) into the collectingtank V_(K) is lowered below a predetermined lower pH command by theregenerant fluid “breaking through” the main cation exchanger I_(X) whenthe concentrate fluid in the main cation exchanger I_(X) is completelydisplaced by the regenerant fluid.

Regeneration Step 2 (Regeneration):

The metal ions bound to the ion exchanger resin of the main cationexchanger I_(X) are adsorbed by the regenerant fluid. H₃O⁺ ions, whichare bound to the cation exchanger resin in lieu of the metal ions,occupy the linkage sites of the ion exchanger material for the metalions. For regeneration, the regenerant fluid can be circulated once orseveral times through the main column I_(X). While the regenerant fluidcontacts the cation exchanger material in the main column I_(X), thecirculation of the concentrate fluid between the collecting tank V_(K)and the main cation exchanger I_(X) is interrupted. The cation exchangerresin can be heated in order to achieve faster regeneration of the maincation exchanger I_(X).

Regeneration Step 3 (Displacement of the Regenerant Fluid):

Once regeneration is completed, the regenerant fluid is again driven outof the collecting tank V_(K) by the concentrate fluid, the regenerantfluid being recirculated back into the first regenerant fluid vesselV_(RS1). The advantage of this manner of proceeding is that the pH valueof the concentrate fluid is not unnecessarily lowered through entrainedregenerant fluid. Like the other method steps, this method step may alsobe automated in that the volume stream of concentrate fluid flowing fromthe collecting tank V_(K) to the main cation exchanger I_(X) (V_(ix))and from there into the first regenerant fluid vessel V_(RS1), iscontrolled through automated valve switching for a set time or forexample through measuring the pH value at the transition between themain cation exchanger I_(X) and the first regenerant fluid vesselV_(RS1). In the latter case, pH sensors may also detect whether the pHvalue of the regenerant fluid flowing from the main cation exchangerI_(X) is raised above a predetermined upper pH command by theconcentrate fluid “breaking through” the main cation exchanger I_(X)when the regenerant fluid in the main cation exchanger I_(X) iscompletely displaced by the concentrate fluid.

In order to achieve a continuous manner of proceeding, several maincation exchangers I_(X) may be provided, concentrate fluid flowingthrough said cation exchangers at different times. Through the maincation exchangers I_(X) through which the concentrate fluid is notcirculated is now circulated the regenerant fluid for regenerationthereof, the above mentioned method steps 1, 2 and 3 being preferablyperformed. Accordingly, two main cation exchangers I_(X) may for examplebe provided for, the concentrate fluid being constantly circulatedthrough the one exchanger for removing the metal ions from theconcentrate fluid while the other one is being regenerated. Uponcompletion of regeneration, concentrate fluid may displace regenerantfluid in said second main cation exchanger I_(X) according to methodstep 3, transferring it into the first regenerant fluid vessel V_(RS1).Concurrently, the regenerant fluid can displace the concentrate fluid inthe first main cation exchanger I_(X) according to method step 1 so thatsaid exchanger can be regenerated next.

To further optimize the process, the metal ion concentration in theconcentrate fluid can be lowered further so that it can be directly fedinto effluent collecting assemblies without having to be subjected tofurther waste water treatment.

The maximum concentration of the metal ions needed for this purpose mustgenerally be below 1 ppm. The following further optional method stepsserve this purpose.

Method Step 4 (Washing):

As water and ions are permanently transferred from the diluatecompartments into the concentrate compartments of the electrodialysisequipment E and as the concentrate fluid is dosed with NaOH solution,the volume of the concentrate fluid steadily increases. Therefore,concentrate fluid from the electrodialysis arrangements E is collectedin a service reservoir V_(ZK), at least to the extent of this increasein volume, while the main cation exchanger I_(X) is being charged(method step 6) and regenerated (method steps 1, 2 and 3). Said servicereservoir V_(ZK) is coupled to the collecting tank V_(K) and to the maincation exchanger I_(X).

In order to treat the concentrate fluid held in the service reservoirV_(ZK) in such a manner that it may be directly fed to the effluentcollecting assemblies, the metal concentration must be less than 1 ppm.If the concentrate fluid stored in the service reservoir V_(ZK) weretreated in the main cation exchanger I_(X) directly after method step 3,it would not be possible to reliably achieve the low metal concentrationof less than 1 ppm required because the main cation exchanger I_(X) isstill contaminated with concentrate fluid originating from thecollecting tank V_(K). In order to achieve the low metal concentration,the concentrate fluid in the main cation exchanger I_(X) is displaced bywash water and transferred to the collecting tank V_(K) afterregeneration of the main cation exchanger I_(X) (method step 2) andafter the regenerant fluid has been driven out of the main cationexchanger I_(X) by the concentrate fluid (method step 3).

The wash water originating from washing may either be added to theelectroless metal plating bath to complement the volume thereof or becombined to the rinse waters for further processing during operation.

Method Step 5 (Final Treatment of the Concentrate Fluid):

After method step 4, the concentrate fluid stored in the servicereservoir VZK is passed across the main cation exchanger I_(X). Metalions from the concentrate fluid are exchanged for H₃O⁺ ions in theprocess.

Safety cation exchanger I_(S), which are coupled to the main cationexchanger I_(X) for post-treatment of the concentrate fluid treated inthe main cation exchanger I_(X), are further provided. After theconcentrate fluid has been passed through the safety cation exchangerI_(S), it contains metal ions in a concentration of less than 1 ppm sothat it can be directly fed into effluent collecting assemblies.

Method Step 6 (Charging):

After the main cation exchanger I_(X) has been regenerated, it can becoupled again to the collecting tank V_(K). This method step can beperformed either after method step 3—if the optional method steps 4 and5 are not performed—or after method step 5. With concentrate fluid beingagain circulated from the collecting tank V_(K) through the main cationexchanger I_(X), the latter is again charged with metal ions. Thecharging process lasts for about 4-12 hours. The time needed depends onthe design of the main cation exchanger I_(X).

The above mentioned method sequence . . . -6-1-2-3-6- . . . or, as analternative, . . . -6-1-2-3-4-5-6- . . . is periodically repeated for asmany times as are needed for the metal salts present in the regenerantfluid to just no longer crystallize. For the regenerant fluid isenriched in metal salts due to its repeated utilization. In this case,the highest possible metal ion concentration is achieved. This maximumconcentration can be sensed through the number of cycles or by means ofa suited detector such as a photocell or a pH meter.

Once the maximum metal ion concentration is reached, the regenerantfluid is transferred in part or in whole to the regenerant fluid vesselV_(RS1) from where it is fed to the metal plating bath tank M. Theremaining regenerant fluid is enriched in fresh acid and adjusted to asuited pH value.

The safety cation exchanger I_(S) merely serves to make sure that theconditions for feeding the fluid into the effluent collecting assembliesare met and is therefore only charged to a very little extent. As aresult thereof, the regeneration cycles need only seldomly be performedor a quite small amount of ion exchanger resin only is needed in thiscolumn.

The regeneration of the safety cation exchanger Is mounted downstream iscarried out in a manner analogous to that for regenerating the maincation exchanger I_(X). The only difference is that only freshregenerant fluid originating from the first regenerant fluid vesselV_(RS1) or from the second regenerant fluid vessel V_(RS2) andcontaining less than 1 ppm nickel ions is used. The regenerant fluid ispreferably conducted only once through the safety cation exchanger Isbefore it is collected in the vessel V_(RS1) and further used forregenerating the main cation exchanger I_(X). The regenerant fluidremaining in the safety cation exchanger I_(S) is displaced by washwater and also introduced into vessel V_(RS1). The wash water used fordriving the regenerant fluid out of the safety cation exchanger I_(S) isrecirculated back into the metal plating bath so that no additionalwaste water is generated.

For further explanation of the present invention, the way of functioningof a preferably utilized electrodialysis equipment E will be explainedby way of example. In this regard, the reader is referred to FIG. 2:

FIG. 2 schematically illustrates the basic structure of theelectrodialysis arrangements E1 and E2 in the simplest implementation.In the two cases, anodes An and cathodes Ka are housed in thecorresponding anode compartments AR1, AR2 or in the correspondingcathode compartments KR1, KR2. These compartments contain anexchangeable electrolyte solution, preferably a sodium sulfate solution.

The anode or cathode compartments are separated from the adjacentelectrolyte compartments by cation exchange membranes K. Such typemembranes, like the other ion exchange membranes used, are freelyavailable and are sold for example by DuPont de Nemours, U.S.A.

The diluate fluid flows through all of the diluate compartments Dixy(Di1 a, Di2 a, Di2 b) and the concentrate fluid through all of theconcentrate compartments Koxy (Ko1 a, Ko1 b, Ko2 a) since both thediluate compartments Dixy and the concentrate compartments Koxy arehydraulically connected in parallel. This is schematically shown byarrows.

In the electrodialysis arrangement E1 schematically shown in the upperportion of the Figure, the anode compartment AR1 is adjoined with afirst concentrate compartment Ko1 a. The two compartments are separatedby a cation exchange membrane K. The concentrate fluid flows throughconcentrate compartment Ko1 a. On the cathode side, said firstconcentrate compartment is defined by an anion exchange membrane A.Toward the cathode Ka, the concentrate compartment Ko1 a is adjoinedwith a diluate compartment Di1 a through which the diluate fluid iscirculated. On the cathode side, the diluate compartment Di1 a is againadjoined with a concentrate compartment Ko1 b through which theconcentrate solution is circulated. The two compartments are separatedfrom one another by a monoselective cation exchange membrane KS. Theconcentrate compartment Ko1 b is separated from the adjacent cathodecompartment KR1 by a cation exchange membrane K.

Sodium ions contained in the concentrate compartment Ko1 a are nottransferred into the diluate compartment Di1 a. In the case of a typicalnickel-phosphorus plating bath, the diluate solution contains ions ofnickel, sodium, hypophosphite (H₂PO₂ ⁻), orthophosphite (HPO₃ ⁻⁻),sulfate and carboxylic acid (RCOO⁻). All of the anions of the ionspecies contained in the diluate compartment Di1 a, i.e., the ions ofhypophosphite, orthophosphite, sulfate and carboxylic acid, aretransferred to the concentrate compartment Ko1 a through the anionexchange membrane A and the singly charged sodium and H₃O⁺ ions amongthe cations thereof, to the concentrate compartment Ko1 b through themonoselective cation exchange membrane KS. By contrast, the doublycharged nickel ions are not transferred to the concentrate compartmentKo1 b but remain in the diluate compartment. Small concentrations ofhydroxide ions possibly contained in the concentrate compartment Ko1 bcannot pass into the diluate compartment. The same applies to the ionsof hypophosphite, orthophosphite, sulfate and carboxylic acid.

Accordingly, the end result obtained with the electrodialysisarrangement E1 is that all of the anions are transferred to theconcentrate compartment whereas, among the cations, only the sodium andH₃O⁺ ions pass into the concentrate compartment, the nickel ions do not.

In the electrodialysis arrangement E2 schematically shown in the lowerportion of the Figure the anode compartment AR2 is adjoined with a firstdiluate compartment Di2 b. On the cathode side, the anode compartment isdefined by a cation exchange membrane K. The diluate fluid flows throughsaid diluate compartment Di2 b. On the cathode side, the diluatecompartment Di2 b is defined by a monoselective anion exchange membraneAS. On the cathode side of the diluate compartment, a concentratecompartment Ko2 a through which the concentrate fluid is circulatedadjoins. Said compartment is separated from a neighbouring seconddiluate compartment Di2 a by an anion exchange membrane A. The diluatefluid is circulated through said diluate compartment. On the cathodeside, said second diluate compartment Di2 a is separated from theadjoining cathode compartment KR2 by means of a cation exchange membraneK.

Cations are not allowed to pass from the first diluate compartment Di2 binto the adjacent concentrate compartment Ko2 a since the twocompartments are separated from one another by a monoselective anionexchange membrane AS. Likewise, sodium ions contained in the concentratecompartment cannot pass into the second diluate compartment Di2 abecause of an anion exchange membrane A that prevents the transfer ofthe sodium ions. Anions contained in the second diluate compartment Di2a, namely ions of hypophosphite, orthophosphite, sulfate, carboxylicacid and hydroxide are transferred into the central concentratecompartment Ko2 a. Among the anions that have entered the concentratecompartment, but the singly charged anions, namely the ions ofhypophosphite, carboxylic acid and hydroxide, are allowed to passthrough the monoselective anion exchange membrane AS into the diluatecompartment Di2 b.

The end result of the partial processes taking place in thiselectrodialysis arrangement E2 is that the interfering bath constituentsare selectively transferred into the concentrate compartment Ko2 awhereas the active substances are recirculated back into the diluatesolution once they have been passed across the concentrate compartment.

Any number of compartments Ko1 y (Ko1 a, Ko1 b) and Di1 y (Di1 a) on theone hand and Ko2 y (Ko2 a) and Di2 y (Di2 a, Di2 b) on the other handcan preferably be arranged to form a package.

It is understood that the examples and embodiments described herein arefor illustrative purpose only and that various modifications and changesin light thereof as well as combinations of features described in thisapplication will be suggested to persons skilled in the art and are tobe included within the spirit and purview of the described invention andwithin the scope of the appended claims. All publications, patents andpatent applications cited herein are hereby incorporated by reference.

Listing of Numerals

-   M metal plating bath tank-   S rinse water tank-   E, E1, E2 electrodialysis equipment/arrangements-   V_(D) diluate tank-   V_(K) collecting tank-   I_(X) main cation exchanger-   I_(S) safety cation exchanger-   V_(ZK) service reservoir-   V_(RS1) first regenerant fluid vessel-   V_(RS2) second regenerant fluid vessel-   Koxy, Ko1 y, Ko2 y-   Ko1 a, Ko1 b, Ko2 a concentrate compartments-   Dixy, Di1 y, Di2 y-   Di1 a, Di2 a, Di2 b diluate compartments-   An anode-   Ka cathode

1. A device for regenerating an electroless metal plating bath,comprising: a) electrodialysis arrangements (E1, E2), each havingdiluate compartments (Di1 y, Di2 y) for holding the metal plating bath,concentrate compartments (Ko1 y, Ko2 y) that are separated from thediluate compartments (Di1 y, Di2 y) through ion exchange membranes andare intended to hold a concentrate fluid serving to adsorb interferingsubstances that are to be removed from the metal plating bath as well asanodes (An) and cathodes (Ka), and b) main cation exchangers (I_(x)) forremoving metal ions from the concentrate fluid, said cation exchangersbeing coupled to the concentrate compartments (Ko1 y, Ko2 y) in such amanner that the concentrate fluid is allowed to be conducted through themain cation exchangers (I_(x)) and to be recirculated back into theconcentrate compartments (Ko1 y, Ko2 y) by allowing the fluid to becirculated in a first circuit between the concentrate compartments (Ko1y, Ko2 y) by allowing the fluid to be circulated in a first circuitbetween the concentrate compartments (Ko1y, Ko2y) and collecting tanks(V_(k)) and in a second circuit between the collecting tanks (V_(k)) andthe main cation exchangers (I_(x)); c) wherein said electrodialysisarrangements include: i) a first electrodialysis arrangement (E1) havingalternating concentrate compartments (Ko1 y) and diluate compartments(Di1 y) as well as cathodes (Ka) and anodes (An), the diluatecompartments (Di1 y) being each separated on the cathode side thereoffrom a neighboring concentrate compartment (Ko1 y) by a monoselectivecation exchange membrane (KS) and on the anode side thereof from aneighboring concentrate compartment (Ko1 y) by an anion exchangemembrane (A), and ii) a second electrodialysis arrangement (E2) havingalternating diluate compartments (Di2 y) and concentrate compartments(Ko2 y) as well as cathodes (Ka) and anodes (An), the concentratecompartments (Ko2 y) being each separated on the cathode side thereoffrom a neighboring diluate compartment (Di2 y) by anion exchangemembrane (A) and on the anode side thereof from a neighboring diluatecompartment (Di2 y) by a monoselective anion exchange membrane (AS), sothat the metal plating bath can be conducted simultaneously through allof the diluate compartments (Di1 y, Di2 y) in the two electrodialysisarrangements (E1, E2), the arrangements being connected in parallel, andthe concentrate fluid being conducted through all of the concentratecompartments (Ko1 y, Ko2 y) in the two electrodialysis arrangements (E1,E2), and d) wherein said device includes current supplies (S) for thecathodes (Ka) and the anodes (An) of the first electrodialysisarrangement (E1) and of the second electrodialysis arrangement (E2); ande) wherein the volume streams of fluid to be circulated between theelectrodialysis arrangements and the collecting tanks on the one sideand between the collecting tanks in the main cation exchangers on theother side are adjusted independently of each other.
 2. The deviceaccording to claim 1, wherein first regenerant fluid vessels (V_(RS1))for holding regenerant fluid intended for the regenerations of the maincation exchangers (I_(x)) are further provided, said vessels beingcoupled to the main cation exchangers (I_(x)).
 3. The device accordingto claim 1, wherein service reservoirs (V_(ZK)) for holding concentratefluid are further provided, said reservoirs being coupled to thecollecting tanks (V_(K)) and to the main cation exchangers (I_(x)). 4.The device according to claim 1, wherein safety cation exchangers(I_(S)) are further provided, said exchangers being coupled to the maincation exchangers (I_(S)) are further provided, said exchangers beingcoupled to the main cation exchangers (I_(X)) for post-treatment of theconcentrate fluid treated in the main cation exchangers (I_(X)).
 5. Thedevice according to claim 1, wherein second regenerant fluid vessels(V_(RS2)) for holding regenerant fluid intended for the regeneration ofthe safety cation exchangers (I_(S)) are provided.
 6. The deviceaccording to claim 1, wherein first regenerant fluid vessels (V_(RS1))for holding regenerant fluid intended for the regenerations of the maincation exchangers (I_(x)) are further provided, said vessels beingcoupled to the main cation exchangers (I_(x)).
 7. The device accordingto claim 1, wherein service reservoirs (V_(ZK)) for holding concentratefluid are further provided, said reservoirs being coupled to thecollecting tanks (V_(K)) and to the main cation exchangers (I_(x)). 8.The device according to claim 1, wherein safety cation exchangers(I_(S)) are further provided, said exchangers being coupled to the maincation exchangers (I_(S)) are further provided, said exchangers beingcoupled to the main cation exchangers (I_(X)) for post-treatment of theconcentrate fluid treated in the main cation exchangers (I_(X)).
 9. Thedevice according to claim 1, wherein second regenerant fluid vessels(V_(RS2)) for holding regenerant fluid intended for the regeneration ofthe safety cation exchangers (I_(S)) are provided.
 10. A method forregenerating an electroless metal plating bath, comprising: a)conducting the metal plating bath through the respective diluatecompartments (Di1 y, Di2 y) of electrodialysis arrangements (E1,E2) andb) conducting a concentrate fluid, serving to adsorb interferingsubstances that are to be removed from the metal plating bath, throughrespective concentrate compartments (Ko1 y, Ko2 y) of theelectrodialysis arrangements (E1, E2), said concentrate compartmentsbeing separated from the diluate compartments (Di1 y, Di2 y) by ionexchange membranes, c) moreover passing the concentrate fluid throughmain cation exchangers (I_(x)) and recirculating the fluid back into theconcentrate compartments (Ko1 y, Ko2 y) by circulating the concentratefluid in a first circuit between the concentrate compartments (Ko1 y,Ko2 y) and collecting tanks (V_(k)) and in a second circuit between thecollecting tanks and the main cation exchangers (I_(X)). d) wherein themetal plating bath is conducted through diluate compartments (Di1 y) ina first electrodialysis arrangement (E1) comprising alternatingconcentrate compartments (Ko1 y) and diluate compartments (Di1 y) aswell as cathodes (Ka) and anodes (An), the diluate compartments (Di1 y)being each separated on the cathode side thereof from a neighboringconcentrate compartment (Ko1 y) by a mono selective cation exchangemembrane (KS) and on the anode side thereof from a neighboringconcentrate compartment (Ko1 y) by an anion exchange membrane (A), e)wherein the metal plating bath is conducted through diluate compartments(Di2 y) in a second electrodialysis arrangement (E2) comprisingalternating the diluate compartments (Di2 y) and concentratecompartments (Ko2 y) as well as cathodes (Ka) and anodes (An), theconcentrate compartments (Ko2 y) being each separated on the cathodeside thereof from a neighboring diluate compartment (Di2 y) by an anionexchange membrane (A) and on the anode side thereof from a neighboringdiluate compartment (Di2 y) by a monoselective anion exchange membrane(AS), and f) wherein the metal plating bath is simultaneously conductedthrough all of the diluate compartments (Di1 y, Di2 y) in the twoelectrodialysis arrangements (E1, E2), the arrangements being connectedin parallel, and the concentrate fluid being conducted through all ofthe concentrate compartments (Ko1 y, Ko2 y) in the two electrodialysisequipments (E1, E2); and g) wherein the volume streams of fluid to becirculated between the electrodialysis arrangements and the collectingtanks on the one side and between the collecting tanks in the maincation exchangers on the other side are adjusted independently of eachother.
 11. The method according to claim 10, wherein, for regeneratingthe main cation exchangers (I_(x)), concentrate fluid contained in themain cation exchangers (I_(x)) is displaced by a regenerant fluid and isrecirculated back into the collecting tanks (V_(K)), the main cationexchangers (I_(x)) being regenerated in the process.
 12. The methodaccording to claim 11, wherein concentrate fluid flows through severalmain cation exchangers (1 _(X)) at different times with the regenerantfluid being circulated through those main cation exchangers (I_(X))through which the concentrate fluid is not circulating for regenerationthereof.
 13. The method according to claim 11, wherein the regenerantfluid is drawn from first regenerant fluid vessels (V_(RS1)) and istransferred to the main cation exchangers (I_(X)).
 14. The methodaccording to claim 13, wherein the regenerant fluid is displaced by theconcentrate fluid after regeneration of the main cation exchangers(I_(X)) is complete, the regenerant fluid being recirculated back intofirst regenerant fluid vessels (V_(RS1)).
 15. The method according toclaim 13, wherein concentrate fluid flows through several main cationexchangers (1 _(X)) at different times regenerant fluid being circulatedthrough those main cation exchangers (I_(X)) through which theconcentrate fluid is not circulating for regeneration thereof.
 16. Themethod according to claim 11, wherein the regenerant fluid is displacedby the concentrate fluid after regeneration of the main cationexchangers (I_(X)) is complete, the regenerant fluid being recirculatedback into first regenerant fluid vessels (V_(RS1)).
 17. The methodaccording to claim 10, wherein concentrate fluid flows through severalmain cation exchangers (1 _(X)) at different times with the regenerantfluid being circulated through those main cation exchangers (I_(X))through which the concentrate fluid is not circulating for regenerationthereof.
 18. The method according to claim 10, wherein, for regeneratingthe main cation exchangers (I_(x)), concentrate fluid contained in themain cation exchangers (I_(x)) is displaced by a regenerant fluid and isrecirculated back into the collecting tanks (V_(K)), the main cationexchangers (I_(X)) being regenerated in the process.